Threshold matrix, storage unit for storing threshold matrix as data, and raster image processor incorporating storage unit

ABSTRACT

A normalization threshold side size Nr which serves as an index indicative of how many times the pattern frequency r is repeated in one threshold matrix is defined as Nr=N×r/R where N represents a size of one side of the threshold matrix as the number of pixels, and the threshold array of the threshold matrix is determined such that the normalization threshold side size Nr is of an value greater than 65.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a threshold matrix for generating ascreen which is called an FM screen or a stochastic screen forconverting a continuous-tone image into a dot pattern representative ofa binary image, and more particularly to a threshold matrix suitable foruse in a printing-related apparatus (output system) such as afilmsetter, a platesetter, a CTP (Computer To Plate) apparatus, a CTC(Computer To Cylinder) apparatus, a DDCP (Direct Digital Color Proof)system, etc., an ink jet printer, or an electrophotographic printer.

2. Description of the Related Art

Heretofore, so-called AM (Amplitude Modulation) screens characterized byscreen ruling, screen angle, and dot shape, and FM (FrequencyModulation) screens have been used in the art of printing.

A process of generating a threshold matrix for FM screens is disclosedin Japanese Laid-Open Patent Publication No. 8-265566.

According to the disclosed process, an array of elements of a thresholdmatrix, i.e., an array of thresholds, is generated in an ascending orderor a descending order by determining threshold positions such that theposition of an already determined threshold is spaced the greatestdistance from the position of a threshold to be newly determined. Thedot pattern of a binary image that is generated using the thresholdmatrix thus produced has dots which are not localized. Even when a dotpattern is generated using a plurality of such threshold matrixes thatare juxtaposed, the dot pattern does not suffer a periodic patternproduced by the repetition of threshold matrixes.

A plurality of patent documents given below are relevant to thegeneration of a threshold matrix.

Japanese Patent No. 3400316 discloses a method of correcting halftoneimage data by extracting a pixel having a weakest low-frequencycomponent of a certain dot pattern, from white pixels (unblackenedpixels), and a pixel having a strongest low-frequency component of thedot pattern, from blackened pixels, and switching around the extractedwhite and blackened pixels. Thus, the dot pattern is intended to besmoothed or leveled.

Japanese Laid-Open Patent Publication No. 2001-292317 reveals a processof determining threshold positions in a threshold matrix such that anext blackened pixel is assigned to a position having a weakestlow-frequency component of the threshold matrix.

Japanese Laid-Open Patent Publication No. 2002-368995 shows a process ofdetermining threshold positions in a threshold matrix such that when anarray of thresholds in the threshold matrix has been determined up to acertain gradation and a threshold position for a next gradation is to bedetermined, blackened pixels are assigned to positions for notstrengthening a low-frequency component.

Japanese Laid-Open Patent Publication No. 2002-369005 discloses aprocess of generating a threshold matrix according to the process shownin Japanese Patent No. 3400316 or Japanese Laid-Open Patent PublicationNo. 2001-292317, based on an ideal dot pattern at a certain gradationwhich is given.

When an FM screen is used for offset printing, it causes shortcomings inthat the quality of printed images suffers some grainness. FM screensalso cause disadvantages in that a dot gain tends to become large andimages are reproduced unstably when images are printed, or when filmsare output in an intermediate printing process, or when a printing plateis output by a CTP apparatus.

According to the conventional FM screening process, when a dot size isdetermined to be the size of a dot made up of one pixel or a dot made upof four pixels according to a 1 (1×1)-pixel FM screen or a 4 (2×2)-pixelFM screen, an array of thresholds of a threshold matrix is determined byan algorithm for generating FM screens, thus determining an outputquality, and only the dot size serves as a parameter for determining thequality of FM screens. For example, if a dot size is determined to be a3×3-pixel FM screen dot size with respect to an output system which isincapable of stably reproducing 2×2-pixel FM screen dots for highlightareas, then the resolution (referred to as pattern frequency or patternresolution) for intermediate tones is lowered, resulting in a reductionin the quality of images.

FIG. 25 of the accompanying drawings shows a conventional dot pattern 1in a highlight area where the dot percentage of a 2×2-pixel FM screen is5%, a conventional dot pattern 2 in an intermediate tone area where thedot percentage of the 2×2-pixel FM screen is 50%, a conventional dotpattern 3 in a highlight area where the dot percentage of a 3×3-pixel FMscreen is 5%, and a conventional dot pattern 4 in an intermediate tonearea where the dot percentage of the 3×3-pixel FM screen is 50%.

FIG. 26 of the accompanying drawings shows a power spectrum generatedwhen the dot pattern 2 of the 2×2-pixel FM screened shown in FIG. 25 isFFTed (Fast-Fourier-Transformed), and FIG. 27 of the accompanyingdrawings shows a power spectrum generated when the dot pattern 4 of the3×3-pixel FM screen shown in FIG. 25 is FFTed.

In FIG. 25, at the dot percentage of 50% in the intermediate tone area,the dot pattern 2 of the 2×2-pixel FM screen suffers less grainness thanthe dot pattern 4 of the 3×3-pixel FM screen, but has the dot percentageless reproducible in the printed image. On the other hand, at the dotpercentage of 50% in the intermediate tone area, the dot pattern 4 ofthe 3×3-pixel FM screen has a pattern frequency 6 of about 13 c/mm whichis lower than the pattern frequency 5 of about 20 c/mm of the dotpattern 2 of the 2×2-pixel FM screen. The pattern frequencies 5, 6 whichare of peak values are also called a peak spatial frequency fpeak.

The output resolution of an output system such as an imagesetter, a CTPapparatus, etc. (the output resolution of an output system willhereinafter be referred to as output resolution R) is set to 2540pixels/inch=100 pixels/mm or 2400 pixels/inch=94.488 pixels/mm, forexample. With those settings, the dot size of the 1×1 pixel FM screen is10 μm×10 μm (10.6 μm×10.6 μm), and the dot size of the 2×2 pixel FMscreen is 20 μm×20 μm (21.2 μm×21.2 μm). In this description, the outputresolution R is different from the pattern frequencies 5, 6 of the dotpatterns 2, 4 shown in FIGS. 26, 27.

As described above, only the dot size is a parameter for determining thequality of conventional FM screens.

It is considered in this connection that the conventional FM screenshave a greater periphery length (to be described in detail later on)than the AM screens at the same dot percentage.

Based on the concept that N×M pixels (usually N=M) constitute a unit forreproducing a gradation, a threshold matrix is made up of an array ofN×M thresholds corresponding to the N×M pixels. A plurality of suchthreshold matrixes (also called as dither matrixes) are repetitivelyarrayed for comparison with continuous-tone image data. Accordingly,undesirable unevenness or irregularity of hue or shade in image tends tooccur at a pitch or an angle due to the repeated units of the thresholdmatrix.

For example, if 100×100 threshold matrixes corresponding to 100×100pixels are obtained with respect to an output system whose resolution Ris 2540 pixels/inch=100 pixels/mm, then grid-like unevenness (noise) ata pitch of 1 mm or unevenness inclined at 45 degrees at a pitch of about0.71 mm are liable to occur in output images.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a threshold matrixwhich will solve the problems of the conventional FM screens, is capableof freely setting dots of a minimum size and a pattern frequency in anintermediate gradation, is optimum for use in an output system, and iscapable of reproducing high-quality images of excellent printability.

Another object of the present invention is to provide a threshold matrixwhich is capable of reducing undesirable unevenness of hue or shadecaused by a repetition of threshold matrixes.

Still another object of the present invention is to provide a thresholdmatrix which is capable of reducing the undesirable unevenness of hue orshade caused by the size (the number of constituent thresholds) of thethreshold matrix.

According to the present invention, there is provided a threshold matrixfor converting a continuous-tone image into a dot pattern representing abinary image, wherein a minimum number of pixels making up dots of thedot pattern is 2×2, when a reference periphery length proportion perunit area of the dot pattern is defined as Ref_sur=(4×r×Q^(1/2))/R wherer represents a pattern frequency, Q represents a blackened ratio, and Rrepresents an output resolution, and a periphery length proportiondetermined per unit area for the dot pattern generated by the thresholdmatrix corresponding to the blackened ratio Q is represented by Mes_sur,a threshold array of the threshold matrix is determined such that a dotpattern is generated where a dot pattern periphery length evaluationindex defined as Mes_sur/Ref_sur does not exceed 1.14 for all blackenedratios Q ranging from 0 to 1, and that a normalization threshold sidesize Nr which serves as an index indicative of how many times thepattern frequency r is repeated in one threshold matrix is defined asNr=N×r/R where N represents a size of one side of the threshold matrixas a number of pixels, and the normalization threshold side size Nr isof an value greater than 65.

According to the present invention, there is provided a threshold matrixfor converting a continuous-tone image into a dot pattern representing abinary image, wherein when a reference periphery length proportion perunit area of the dot pattern is defined as Ref_sur=(4×r×Q^(1/2))/R wherer represents a pattern frequency, Q represents a blackened ratio, and Rrepresents an output resolution, and a periphery length proportiondetermined per unit area for the dot pattern generated by the thresholdmatrix corresponding to the blackened ratio Q is represented by Mes_sur,a threshold array of the threshold matrix is determined such that a dotpattern is generated where a dot pattern periphery length evaluationindex defined as Mes_sur/Ref_sur does not exceed 1.14 for all blackenedratios Q ranging from 0 to 1 is generated, and that a normalizationthreshold side size Nr which serves as an index indicative of how manytimes the pattern frequency r is repeated in one threshold matrix isdefined as Nr=N×r/R where N represents a size of one side of thethreshold matrix as a number of pixels, and the normalization thresholdside size Nr is of an value greater than 65, but not in excess of 75.

According to the present invention, the threshold matrix allows dots ofa minimum size and a pattern frequency in an intermediate tone area tobe established freely, is optimum for use with an output system, and canreproduce high-quality images of excellent printability.

According to the present invention, the undesirable unevenness of hue orshade due to repeated units of the threshold matrix can be reduced.

According to the present invention, the undesirable unevenness of hue orshade due to the size (the number of constituent thresholds) of thethreshold matrix can be reduced.

According to the present invention, the periphery length of dots can bereduced with respect to the resolution of a dot pattern in anintermediate tone area.

Specifically, the threshold matrix is capable of generating an imagewhere dots are reliably assigned to a highlight area, and grainness isreduced and a dot gain is small in an intermediate tone area.

The threshold matrix is stored as data in a storage unit.

The storage unit which stores the threshold matrix as data isincorporated in a raster image processor.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a threshold matrix generating system towhich a process of generating a threshold matrix according to anembodiment of the present invention is applied;

FIG. 2 is a flowchart of an overall sequence of the process ofgenerating a threshold matrix which is carried out by the thresholdmatrix generating system shown in FIG. 1;

FIG. 3A is a diagram of a white noise pattern generated at a dotpercentage of 50% by 1×1 pixel FM-screened dots;

FIG. 3B is a diagram showing an FFT process and a bandpass filteringprocess on the white noise pattern;

FIG. 3C is a diagram showing an IFFT-processed space-domain imageconverted from the frequency-domain image shown in FIG. 3B;

FIG. 3D is a diagram showing a binary image converted from thespace-domain image shown in FIG. 3C;

FIG. 4 is a diagram showing the number of dots vs. dot percentage;

FIG. 5A is a diagram showing a periphery length of small dots;

FIG. 5B is a diagram showing a periphery length of large dots at thesame dot percentage as with FIG. 5A;

FIG. 6 is a diagram showing a regular dot pattern used to illustrate adot pattern periphery length evaluation index;

FIG. 7A is a diagram showing a pattern frequency curve having a singlepeak;

FIG. 7B is a diagram showing another pattern frequency curve having asingle peak;

FIG. 7C is a diagram showing a pattern frequency curve having two peaks;

FIG. 7D is a diagram showing another pattern frequency curve having twopeaks;

FIG. 7E is a diagram showing a pattern frequency curve without peaks;

FIG. 8 is a diagram showing periphery length evaluation indexes for dotpatterns generated by the conventional threshold matrix and thethreshold matrix according to the present embodiment;

FIG. 9 is a diagram illustrative of a process of generating dots with areduced periphery length;

FIG. 10A is a diagram showing a pixel position for reducing a peripherylength;

FIG. 10B is a diagram showing another pixel position for reducing aperiphery length;

FIGS. 11A and 11B are diagrams showing pixel positions for reducing thedegree of variations of a periphery length in the direction of ascanning line;

FIG. 12A is a diagram showing the periphery length ratio of a dotpattern according to the embodiment;

FIG. 12B is a diagram showing the periphery length ratio of aconventional dot pattern;

FIG. 13 is a diagram showing, by way of comparison, the standarddeviation of the periphery length ratio of the dot pattern according tothe embodiment and the standard deviation of the periphery length ratioof the conventional dot pattern;

FIG. 14 is a flowchart of a detailed sequence of a threshold positiondetermining process in step S4 of the overall sequence shown in FIG. 2;

FIG. 15 is a diagram illustrative of a process of determining athreshold position for a next gradation;

FIG. 16A is a diagram showing threshold candidate positions;

FIG. 16B is a diagram showing smallest-size dots placed in the thresholdcandidate positions;

FIG. 17A is a diagram showing a dot pattern having 2×2-pixel dots of aminimum size and a dot percentage of 30%;

FIG. 17B is a diagram showing a pattern with stressed dark and lightareas which is produced by processing the dot pattern shown in FIG. 17Awith a visual characteristic filter;

FIG. 17C is a diagram showing a dot pattern of a conventional 2×2-pixeldot FM screen;

FIG. 17D is a diagram showing a pattern with stressed dark and lightareas which is produced by processing the dot pattern shown in FIG. 17Cwith a visual characteristic filter;

FIG. 18A is a perspective view of the pattern of dark and light areasshown in FIG. 17B;

FIG. 18B is a perspective view of the pattern of dark and light areasshown in FIG. 17D;

FIG. 19A is a diagram showing a dot pattern having a dot percentage of10% which is generated by the threshold matrix according to theembodiment;

FIG. 19B is a diagram showing a dot pattern having a dot percentage of20% which is generated by the threshold matrix according to theembodiment;

FIG. 19C is a diagram showing a dot pattern having a dot percentage of30% which is generated by the threshold matrix according to theembodiment;

FIG. 19D is a diagram showing a dot pattern having a dot percentage of40% which is generated by the threshold matrix according to theembodiment;

FIG. 19E is a diagram showing a dot pattern having a dot percentage of50% which is generated by the threshold matrix according to theembodiment;

FIG. 19F is a diagram showing a dot pattern having a dot percentage of70% which is generated by the threshold matrix according to theembodiment;

FIG. 20 is a diagram showing unevenness visibility of a normalizedthreshold side size;

FIG. 21 is a graph showing unevenness visibility with respect to anormalized threshold side size;

FIG. 22 is a diagram showing a square block process;

FIG. 23A is a diagram showing a supercell with a Utah block (brickblock);

FIG. 23B is a diagram showing the manner in which the supercell isdeveloped into a Utah block;

FIG. 23C is a diagram showing a repetitive pattern of Utah blocks;

FIG. 23D is a diagram showing a brick block pattern;

FIG. 24 is a block diagram of a printing/platemaking systemincorporating threshold matrixes generated by a threshold matrixgenerating apparatus;

FIG. 25 is a diagram showing dot patterns at dot percentages of 5% and50% of 2×2 pixel FM-screened dots and dot patterns at dot percentages of5% and 50% of 3×3 pixel FM-screened dots according to conventional art;

FIG. 26 is a diagram showing a power spectrum generated when the dotpattern at the dot percentage of 50% of the 2×2 pixel FM-screened dotsis processed by FFT; and

FIG. 27 is a diagram showing a power spectrum generated when the dotpattern at the dot percentage of 50% of the 3×3 pixel FM-screened dotsis processed by FFT.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a basic arrangement of a threshold matrix generating system10 according to an embodiment of the present invention.

As shown in FIG. 1, the threshold matrix generating system 10 has animage data generator 12 for generating image data I including a testpattern of uniform density and also generating a two-dimensional address(x, y) of the image data I, a threshold matrix storage unit 14 forstoring a plurality of threshold matrixes TM and outputting a thresholdth read by the address (x, y), a comparator 16 for comparing thethreshold th and the image data I and outputting binary image data H, athreshold matrix generating apparatus 20 including a dot patterngenerator 18 for generating dot pattern data Ha corresponding to thebinary image data H output from the comparator 16, the threshold matrixgenerating apparatus 20 serving to determine a threshold array(threshold positions) of the threshold matrixes TM such that a dotpattern represented by the dot pattern data Ha will be a desired dotpattern, and an output system 22 for forming the dot patterncorresponding to the dot pattern data Ha on a film, a printing plate PP,or a printed material.

The threshold matrix storage unit 14 comprises a recording medium suchas a hard disk or the like. The image data generator 12, the comparator16, the dot pattern generator 18, and the threshold matrix generatingapparatus 20 may comprise function realizing means that are achievedwhen a program stored in a personal computer (including a CPU, a memory,an input unit 20 a such as a keyboard, a mouse, etc., and an output unitsuch as a display unit 20 b, a printer 20 c, etc.) is executed by thecomputer. The function realizing means of the threshold matrixgenerating apparatus 20 may comprise a piece of hardware. An arrangementand operation of the function realizing means of the threshold matrixgenerating apparatus 20 will be described later on.

In the present embodiment, the output system 22 basically comprises aCTP apparatus having an exposure unit 26 and a drum 27 with printingplate materials EM wound thereon. The exposure unit 26 applies aplurality of laser beams (recording beams), which are turned on and offfor each pixel depending on the dot pattern data Ha, to the printingplate materials EM on the drum 27 that is being rotated in a mainscanning direction MS by a main scanning motor (not shown) at a highspeed, while the exposure unit 26 is being moved in an auxiliaryscanning direction AS along the axis of the drum 27 by an auxiliaryscanning motor (not shown). At this time, a dot pattern representing atwo-dimensional image as a latent image is formed on each of theprinting plate materials EM. The laser beams applied to the printingplate materials EM may be in several hundred channels.

The printing plate materials EM (usually, four printing plate materialswith different screen angles for C, M, Y, K printing plates) on whichthe dot patterns are formed as latent images are developed by anautomatic developing machine 28, producing printing plates PP withvisible dot patterns formed thereon. The produced printing plates PP aremounted on a printing press (not shown), and inks are applied to themounted printing plates PP.

The printing plate materials EM contain a photosensitive material whichshould preferably be a positive image recording material including analkaline dissolvable binder, a substance for generating heat uponabsorption of an infrared radiation or a near-infrared radiation, and athermally decomposable substance for substantially lowering thedissolvability of the binder when not thermally decomposed, as disclosedin Japanese Patent No. 3461377. The printing plates PP should preferablybe made of an image recording material including a photosensitivematerial which comprises a support base such as an aluminum sheet, apolyester film, or the like, and a layer including the above substancesand mounted on the support base.

The alkaline dissolvable binder contains a phenolic resin, an acrylicresin, or a polyurethane resin. The substance for generating heat uponabsorption of an infrared radiation or a near-infrared radiationcomprises a dye, a pigment, or carbon black. The thermally decomposablesubstance for substantially lowering the dissolvability of the binderwhen not thermally decomposed comprises onium salt, diazonium salt, or asubstance containing a quinone diazide compound.

When the inks applied to the printing plates PP are transferred to aprinting sheet as a recording medium such as a photographic sheet or thelike, a desired printed material comprising an image formed on theprinting sheet is obtained.

The output system 22 is not limited to the scanning exposure apparatusemploying laser beams, but may be an apparatus for forming an image on afilm, a printing plate, or a printed material according to a planarexposure process or an ink jet process, or a CTC printing machine.

The threshold array of the threshold matrixes TM stored in the thresholdmatrix storage unit 14 can be recorded and carried around in a portablerecording medium which is a packaged medium such as a DVD, a CD-ROM, aCD-R, a semiconductor memory, or the like.

A process of generating a threshold matrix using the threshold matrixgenerating system shown in FIG. 1 will be described below with referenceto a flowchart of FIG. 2. The process shown in FIG. 2 is based on aprogram which is mainly executed by the threshold matrix generatingapparatus 20.

In step S1 shown in FIG. 2, three parameters are set. The firstparameter represents the size of a threshold matrix TM to be stored inthe threshold matrix storage unit 14, i.e., the size N×N of a thresholdmatrix TM which contains N×N thresholds corresponding to N×N pixels. Thethreshold matrix TM contains thresholds th ranging from 0 to thmax atrespective positions (elements) determined by addresses (x, y). Themaximum threshold thmax has a value that is set to “255” for a systemhaving 8-bit gradations and “65535” for a system having 16-bitgradations. The size N×N of a square threshold matrix will be describedbelow. However, the present invention is also applicable to the size N×Mof an elongate rectangular threshold matrix. Actually, a plurality ofthreshold matrixes TM having the same threshold array and-matrix sizeN×N and laid out as tiles (referred to as a superthreshold matrix STM)are used depending on the size of an image to be processed. Thethresholds th of the threshold matrix TM is determined in view of thethreshold array of the entire superthreshold matrix STM.

Unevenness or irregularity of hue or shade which tends to occur at apitch or an angle due to the repeated units of the threshold matrix TMis less visually perceptible as the size N×N of each of the thresholdmatrixes TM is larger, and is not visually perceptible if a thresholdmatrix TM having the same size as the magnitude (size) ofcontinuous-tone image data to be converted into a dot pattern, i.e., athreshold matrix TM having the same size as the number of pixels ofcontinuous-tone image data to be converted into a dot pattern, isavailable. However, the larger the size N×N of a threshold matrix TM,the longer the period of time required to generate the threshold matrixTM, and the greater storage capacity of a recording medium such as amemory or the like required to store the threshold matrix TM. Even ifsuch a large threshold matrix TM can be stored in a recording medium, along calculation time is required by a binary conversion process usingthe threshold matrix TM. Therefore, such a large threshold matrix TM isnot practical in use.

For the above reasons, the size N×N of a threshold matrix TM shouldpreferably be set to an optimum size that is not too large in view ofunevenness caused due to the repeated units of a threshold matrix. Theoptimum size of a threshold matrix TM will be described later on.

In the present embodiment, the size of a pixel that can be output fromthe output system 22 is represented by 10 μm×10 μm, which corresponds toa 1×1-pixel dot or 1 pixel. The size 10 μm×10 μm is a minimum unit thatcan be controlled by the exposure unit 26 for recording image data onthe printing plate materials EM.

The second parameter represents the number of pixels that make up a dotof a minimum size which can stably be output from the output system 22,or stated otherwise, can stably be formed on the printing plates PPwhich are output from the output system 22. The dot of a minimum sizemay be set to a 1-pixel dot (the number of pixels that make up a dot ofa minimum size is one), a 2-pixel dot, a 3-pixel dot, a 2×2-pixel (thenumber of pixels that make up a dot of a minimum size is four) dot, a2×3-pixel (6-pixel) dot, a 3×3-pixel (9-pixel) dot, etc. In the presentembodiment, it is assumed that a dot of a minimum size that can stablybe formed on the printing plates PP (in reality, the printed material)is a 2×2-pixel dot whose dot size is represented by 2×2=4 pixels.

The third parameter represents the pattern frequency at a predetermineddot percentage (also referred to as density percentage) in intermediatetones having a dot percentage in the range from 10% to 50%, i.e., thepattern frequency r of an intermediate tone dot pattern. The patternfrequency r of an intermediate tone dot pattern represents the peakspatial frequency fpeak c/mm of a dot pattern in an intermediate tone.

In reality, the peak spatial frequency fpeak is concerned with thereproduction of image details, and also affects image quality in termsof grainness. In the present embodiment, the pattern frequency r is setto a visually sufficiently small value of 20 c/mm, i.e., 508 (20×25.4)LPI (Line Per Inch) (fpeak=r=20 c/mm).

In step S2, a dot candidate position in a highlight area HL and a dotcandidate position in a shadow area SD are determined to provide thepattern frequency r in an intermediate tone.

First, as shown in FIG. 3A, a white noise generator 30 generates a whitenoise pattern WH at a dot percentage of 50% having the same size N×N asthe size N×N of the threshold matrix TM. The white noise pattern WH isan image where 1-pixel dots are randomly positioned in a spatial domain.The white noise pattern WH can be generated so as to have desired valuesin an intermediate tone having a dot percentage in the range from 10% to90%.

Second, the white noise pattern WH is FFTed by an FFT (Fast FourierTransform) unit 32, and then subjected to a bandpass filtering processat the pattern frequency r (±Δ) by a pattern frequency bandpass filter(pattern frequency BPF) 34, producing ring-shaped frequency-domain dataAFFT2 having a radius equal to the pattern frequency r, as shown in FIG.3B.

Third, the frequency-domain data AFFT2 is IFFTed by an IFFT (InverseFast Fourier Transform) unit 36, producing space-domain data A2 of acontinuous-tone image, as shown in FIG. 3C.

Fourth, the value of each of the pixels of the spatial-domain data A2 iscompared with a central gradation value (e.g., 127 if the maximumgradation is 255) by a comparator 38, generating binary data A2_bin, asshown in FIG. 3D.

Of the binary data A2_bin, blackened portions (areas) serve as dotcandidate positions in highlight areas HL and white portions (areas)serve as dot candidate positions in shadow areas SD.

The binary data A2_bin represent candidate positions for placing dots inhighlight areas HL or the shadow areas SD. The pattern of the binarydata A2_bin may not necessarily be produced when the dot percentage is50%. When the binary data A2_bin do not actually represent an optimum50% dot pattern, the pattern may be changed for achieving the optimumdot pattern.

However, a 50% dot pattern can be established when a characteristic dotpattern is to be used at the dot percentage of 50% or when the dotpattern corresponding to the binary data A2_bin can be corrected into anoptimum 50% dot pattern.

Then, in step S3, the number Dn of dots of a minimum size (also referredto as the number of dots of a new minimum size dots or the number of newdots of a minimum size) to be newly set at a present dot percentage isdetermined with respect to the dot percentage for which a dot patternhas been determined. The number Dn(P) of new dots of a minimum size tobe established at each dot percentage P % is expressed asDn(P)=Ds(P)−Ds(P−1) where Ds(P) represents the number of accumulateddots (accumulated values) at each dot percentage P.

Specifically, in step S3, when candidate positions for dots aresuccessively determined as the dot percentage is incremented, the numberDn(P) of dots of a minimum size to be newly established at a present dotpercentage P is determined with respect to the preceding dot percentageP−1 for which a dot pattern has already been determined.

When a dot pattern has a dot percentage P with respect to the size N×Nof a threshold matrix TM, the total number of blackened pixels in thedot pattern corresponding to the size N×N of the threshold matrix TM iscalculated as N×N×P/100. If all the dots of a dot pattern comprise onlydots of a minimum size as 2×2 (n=4)-pixel dots, then since the number ofdots of a minimum size at each dot percentage P is expressed asDs(P)=(N×N×P/100)/n, it is given as (N×N×P/100)/n (n=4), as indicated bya solid straight curve na in FIG. 4, for example.

At this time, the number Dn(P) of dots of a minimum size to be newlyestablished at each dot percentage P is expressed asDn(P)=Ds(P)−Ds(P−1)=(N×N/100)/n.

The vertical axis of the graph shown in FIG. 4 represents a calculatedaccumulated value Ds of the number Dn of dots of a minimum size to benewly established (the number of new dots). Actually, as the dotpercentage P becomes greater than 25%, since adjacent dots of a minimumsize become closer to each other, the actual number of dots in a dotpattern is smaller than the accumulated value of the number Dn of newdots shown in FIG. 4.

If the number Dn of new dots are determined at each dot percentageaccording to the solid straight curve na in FIG. 4 which represents theaccumulated value of the number Dn of new dots, then the thresholdmatrix produces a conventional FM screen, which causes disadvantages inthat a dot gain tends to become large and images are reproduced unstablywhen images are printed or films are output in an intermediate printingprocess.

According to an embodiment of the present invention, in view of the factthat the pattern frequency is low in highlight areas where the dotpercentage is less than 10%, all dots comprise dots of a minimum size inthose highlight areas. In intermediate tone areas where the dotpercentage ranges from 10% to 50%, the size of dots is increased fromthe minimum size, e.g., dots composed of 5 pixels (2×2+1) or more areused. Specifically, in a dot percentage range from 10% to 25%, thenumber Dn of new dots to be established at each dot percentage isgradually reduced, as indicated by a broken-line curve nc whichrepresents the accumulated value of the number of new dots. In a dotpercentage range from 25% to 50%, the number Dn of new dots to beestablished at each dot percentage is set to zero. Alternatively, thenumber Dn is gradually increased, as indicated by the dot-and-dash-linecurve nb which represents the accumulated value of the number of newdots.

In the present embodiment, since the output resolution R of the outputsystem is 100 pixels/mm or 10 μm/pixels, and the pattern frequency r ofthe intermediate tone dot pattern is r=20 c/mm, each side of theN×N-pixel area has to contain 20 blackened dots (one dot comprises 2×2pixels with r c/mm) of a minimum size, each composed of 4 pixels per 100pixels/mm (R pixels/mm). In terms of the size of the N×N-pixel thresholdmatrix TM, the accumulated value Ds of the number Dn of new dots up tothe intermediate tone areas is represented by(N/(R/r))²=N×N×(r/R)²=N×N×(20/100)²=N×N×0.04.

With the above settings, in the intermediate tone areas where the dotpercentage ranges from 10% to 50%, the total number of pixels of a dotpattern generated by the threshold matrix TM at each dot percentage isthe same as with the conventional FM screens, i.e., the dot percentageis the same, but the number of dots is smaller than with theconventional FM screens. Therefore, a periphery length representing thesum of the lengths of the peripheries of all the dots of the dot patternis smaller than with the conventional FM screens.

In the present description, the periphery length (also referred to as adot pattern periphery length) represents the sum of the lengths ofwhite/black boundaries per unit area of a dot pattern. It is known inthe art that the periphery length is correlated to the dot gain and theoutput/printing stability.

For example, as can be seen from dot patterns 100, 104 having the samearea shown in FIGS. 5A and 5B, the dot pattern 100 contains eight1×1-pixel dots 102 each having a periphery length 4L where L representsthe length of one side of each dot, and the dots 102 have a total area8L² and a periphery length 32L. The dot pattern 104 contains two2×2-pixel dots 106 each having a periphery length 8L where 2L representsthe length of one side of each dot, and the dots 106 have a total area8L² and a periphery length 16L. Though the total area of the dots 102 ofthe dot pattern 100 and the total area of the dots 106 of the dotpattern 104 are the same as each other, the dot patterns 100, 104 havedifferent periphery lengths. Stated otherwise, though the dot pattern100 and the dot pattern 104 have the same dot percentage, the sum of thelengths of white/black boundaries per unit area of the dot pattern 104,i.e., the dot periphery length of the dot pattern 104, is one-half ofthe periphery length of the dot pattern 100.

When the dot pattern 100 having the periphery length 32L is varied by adot gain to have its dot percentage increased by +10%, the dot pattern104 having the periphery length 16L is expected to have its dotpercentage increased by +5%.

Various variations indicative of stability such as a dot gain areconsidered to be essentially proportional to the periphery length of thedot pattern.

Generally, if a dot pattern of a higher resolution is generated, thenthe number of pixels making up each dot is reduced, and the peripherylength is increased, resulting in poorer stability.

Therefore, provided that two dot patterns have the same resolution, oneof the dot patterns which has a smaller (shorter) dot periphery lengthis a higher-performance dot pattern. There has not been available ageneral index for evaluating the dot pattern performance based on thedot periphery length. According to the present invention, a dot patternperiphery length evaluation index (also referred to as a dot patternevaluation index) is defined based on a value in AM screens to specify adot pattern, as follows.

When the resolution of a dot pattern (also referred to as a patternresolution or a pattern frequency) is represented by r c/mm, theresolution of the output system by R pixels/mm, and the blackened ratioof the dot pattern by Q (Q=P/100), a regular dot pattern 52 shown inFIG. 6 is assumed.

Each side of each square pattern 54 in the dot pattern 52 with thepattern frequency r has a length 1/r mm. R/r, which is the product ofthe length 1/r mm and the output resolution R pixels/mm, represents thenumber of pixels per side length. If the scanning exposure process isemployed, then R/r represents the number of raster lines (scanninglines) per side length.

If the square pattern 54 has a blackened ratio Q, then the area(occupied area) of a dot 56 in the square pattern 54 is represented by(1/r)²×Q mm². With respect to a dot whose blackened ratio Q is 0.5 ormore, the whitened ratio Q′ (Q′=1−Q) of a whitened area in the squarepattern 54 is used to characterize the dot.

If it is assumed that the dot 56 is of a square shape, then the lengthof each side of the dot 56 is represented by (1/r)×Q^(1/2) mm.

The dot pattern periphery length per unit area (the length of boundarylines of black and white pixels) will be considered in terms of squarepatterns 54 each having an area 1/r×1/r.

In the vertical direction (the main scanning direction) in FIG. 6, theperiphery length of the dot 56 in one square pattern 54 is representedby (1/r)×Q^(1/2)×2 mm. The length of pixels in the vertical direction inone square pattern 54 (also referred to as the length of unit scanninglines) is represented by (R/r) (i.e., “the number of pixels per sidelength or the number of scanning lines per side length”)×(1/r) (i.e.,“the length of one side”)=(R/r)×(1/r) pixels/mm. The dot peripherylength in the length of unit scanning lines in the vertical direction(the main scanning direction) is represented by(1/r)×Q^(1/2)×2/((R/r)×(1/r))=(2×r×Q^(1/2))/R.

Similarly, in the auxiliary scanning direction, the dot periphery length(referred to as a reference periphery length proportion) Ref_sur in theoverall square pattern 54 as a unit area is expressed according to thefollowing equation (1):Ref_sur=(4×r×Q ^(1/2))/R  (1)

It can be seen from the equation (1) that if the output resolution R isconstant, then the dot periphery length is proportional to the patternfrequency r of the dot pattern. Actually, the pattern frequency r of thedot pattern is proportional to the output resolution R. Thus, adimensionless value r/R that indicates the fineness of the dot patternrepresents a numeral affecting the equation (1). However, the outputresolution R and the pattern frequency r of the dot pattern areseparately used so as to be practically applicable with ease.

As shown in FIGS. 26 and 27, the pattern frequency r is judged from thefrequency characteristics produced after the dot pattern is FFTed. Ifthe peak frequency fpeak is clear, as indicated by frequencycharacteristic curves 60, 61 shown in FIGS. 7A and 7B, then the peakfrequency fpeak is used as the pattern frequency r of the dot pattern.

However, if there are two or more frequencies each having an intensityequal to or greater than one-half of the peak frequency fpeak, asindicated by frequency characteristic curves 62, 63 shown in FIGS. 7Cand 7D, then a frequency determined as the weighted mean of the abovefrequencies is used as the pattern frequency r of the dot pattern. Theweighted mean is expressed by the following equation (2):fpeak=(average of Σ(freq×fpower))/(average of Σfpower)  (2)where freq represents a frequency having an intensity equal to orgreater than one-half of the peak frequency fpeak, and fpower representsthe power (intensity) of the frequency. Therefore, fpower is representedas each of hatched areas in FIGS. 7C and 7D.

A dot pattern having a frequency characteristic curve 64 (see FIG. 7E)which has no clear peaks within a frequency range from 0 to R/2 c/mmwhere an analysis after FFT is valid, cannot have a pattern frequency rdefined.

According to an example, it is assumed that the output system 22 has ageneral output resolution R=2400 pixels/inch=94.488 pixels/mm with dotshaving a screen ruling of 175 (the pattern frequency r=6.89 c/mm) and adot percentage P=50% (Q=0.5). The reference periphery length proportionRef_sur which represents a dot periphery length in one dot(corresponding to a dot pattern formed by one threshold matrix) is 0.206as calculated below.

$\begin{matrix}{{Ref\_ sur} = {( {4 \times r \times Q^{1/2}} )/R}} \\{= {( {4 \times 6.89 \times 0.5^{1/2}} )/94.488}} \\{= 0.206}\end{matrix}$

The reference periphery length proportion Ref_sur of 0.206 means thatabout 20 percent of the periphery length of all the pixels making upsquare dots 56 which are arranged at equal intervals at the patternfrequency r constitute black/white boundary lines.

If the screen ruling is 350 (the pattern frequency r=13.78 c/mm), thenthe reference periphery length proportion Ref_sur is 0.412.

The value of the reference periphery length proportion Ref_surrepresents the periphery length (also referred to as an ideal peripherylength) of square dots 56 which are arranged at equal intervals at acertain pattern frequency r (pitch 1/r).

Using the reference periphery length proportion Ref_sur, an evaluationindex for the periphery length of a dot pattern formed by digital datathat can actually be calculated is expressed by the following equation(3):Dot pattern periphery length evaluation index=Mes_sur/Ref_sur  (3)where Mes_sur represents a periphery length proportion per unit areacalculated from a dot pattern actually formed by a digital pattern.

When the dot pattern periphery length evaluation index Mes_sur/Ref_surhas a value of 1, it means that the periphery length of the dot patternis equal to the periphery length of square patterns (square patternsarranged at equal intervals) 54 having the same pattern frequency r. Asthe dot pattern periphery length evaluation index Mes_sur/Ref_sur has asmaller value, the dot pattern has a shorter periphery length withrespect to the pattern frequency r, indicating that the screen isstabler. Unlike AM screens, FM screens are made up of smaller dots.Stated otherwise, FM screens have more scattered dots than AM screens atthe same dot percentage. Therefore, the dot pattern periphery lengthevaluation index Mes_sur/Ref_sur for FM screens is considered to have avalue greater than 1 in most cases.

The dot pattern periphery length evaluation index Mes_sur/Ref_sur forthe dot pattern 2 (see FIG. 25) in the intermediate tone area where thedot percentage of the 2×2-pixel FM screen is 50% will be calculatedbelow.

First, the periphery length proportion Mes_sur of the dots of the dotpattern 2 of the 2×2-pixel FM screen shown in FIG. 25 is calculated asMes_sur=0.6302.

With the dot pattern 2, as shown in FIG. 26, the pattern frequency rcorresponding to the peak spatial frequency fpeak, when calculated tothree significant digits, is r=19.9 c/mm. Therefore, the referenceperiphery length proportion Ref_sur is calculated asRef_sur=(4×r×Q^(1/2))/R=(4×19.9×0.5^(1/2))/94.488=0.5956.

Therefore, the dot pattern periphery length evaluation indexMes_sur/Ref_sur is calculated as Mes_sur/Ref_sur=0.6302/0.5956=1.058.

FIG. 8 shows a characteristic curve 66 representing the dot patternperiphery length evaluation index Mes_sur/Ref_sur for the dot pattern ofthe conventional 2×2-pixel FM screen at each dot percentage, and acharacteristic curve 68 representing the dot pattern periphery lengthevaluation index Mes_sur/Ref_sur for the dot pattern of a 2×2-pixel FMscreen where the dots are of a minimum size, which is generatedaccording to the present embodiment as described later on.

It can be seen from the characteristic curves 66, 68 that sincehighlight areas in a dot percentage range from 0 to 20% and shadow areasin a dot percentage range from 80 to 100% are made up of independentdots such as blackened dots or white dots each comprising 2×2 pixels,i.e., dots comprising only 2×2-pixel dots with no surrounding pixels,the dot pattern periphery length evaluation index Mes_sur/Ref_sur issmall in those dot percentage ranges. At a dot percentage of 50%, thedot pattern periphery length evaluation index Mes_sur/Ref_sur is alsorelatively small.

According to the characteristic curve 66 representing the dot patternperiphery length evaluation index Mes_sur/Ref_sur for the dot pattern ofthe conventional 2×2-pixel FM screen, the dot pattern periphery lengthevaluation index Mes_sur/Ref_sur is large in a dot percentage range from25 to 45% and a dot percentage range from 55 to 75%.

In these dot percentage ranges from 25 to 45% and from 55 to 75%, as canbe seen from the characteristic curve 66, the periphery length is longerwith respect to the pattern frequency r of the dot pattern, indicatingthat the dot pattern includes elements which make output and printedimage data unstable. According to the present embodiment, as can beunderstood from the characteristic curve 68 shown in FIG. 8, the dotpattern periphery length evaluation index Mes_sur/Ref_sur is smaller inthe dot percentage ranges from 25 to 45% and from 55 to 75%.

The dot pattern periphery length evaluation index Mes_sur/Ref_sur shouldpreferably have a value of 1.085 or less. It has been confirmed that thedot pattern periphery length evaluation index Mes_sur/Ref_sur shouldmore preferably have a value of 1.065 or less for greater advantage.

According to the present embodiment, however, if the dot patternperiphery length evaluation index Mes_sur/Ref_sur for a dot pattern isnot in excess of 1.14, then the dot pattern can be used to produce athreshold matrix TM which offers excellent printability and outputcapability by selecting an appropriate threshold matrix size N×N.

The characteristic curves 66, 68 shown in FIG. 8 were plotted based onthe results of calculations for 19 dot patterns for each of thecharacteristic curves 66, 68 in the dot percentage range from 5 to 95%.The calculated 19 dot patterns are sufficient enough to evaluate the dotpattern periphery length evaluation index Mes_sur/Ref_sur. Thisevaluating process is capable of evaluating the dot pattern peripherylength evaluation index Mes_sur/Ref_sur in the entire dot percentagerange from 0 to 100%. Of course, more dot patterns may be calculated toevaluate the dot pattern periphery length evaluation indexMes_sur/Ref_sur.

A process of generating a dot having a smaller periphery length will bedescribed below. According to the process, a certain existing pixel in adot is selected as a blackened pixel candidate if it makes the peripherylength of the dot small based on the information of pixels in thevicinity of the noticed pixel.

For example, as shown in FIG. 9, an existing pixel* is noticed in a dot70 which is made up of nine pixels each representing binary data, andthe sum f(k) of four nearby pixels is determined as f(k)=k1+k2+k3+k4. Ifthe sum f(k) is large, then the noticed pixel* is determined as ablackened pixel candidate, i.e., a candidate position for placing apresent threshold (a threshold for a next gradation) therein.

FIG. 10A shows a pixel cl which is positioned in a dot 72 such that thesum f(k) of four nearby pixels is 4, and FIG. 10B shows a pixel c2 whichis positioned in a dot 73 such that the sum f(k) of four nearby pixelsis 3. These pixels c1, c2 are preferentially selected as thresholdpositioning candidates (blackened pixel candidates), thereby generatingdots with smaller dot periphery lengths.

The above example is simplest in that the periphery length of a dot ismade smaller using the sum f(k) of four pixels in the vicinity of anoticed pixel. Alternatively, a pixel in a dot may be selected as ablackened pixel candidate for making the periphery length of the dotsmaller, by using the information of eight nearby pixels or performingany of various bit pattern matching processes on nearby pixels.

For evaluating a dot pattern, not only an average periphery length perunit area serves as an important index, but also the uniformity of theperiphery length of a dot pattern affects the evaluation. In particular,when the exposure unit 26 relatively or actively scans the drum 27 torecord pixels on the printing plate materials EM wound thereon, if theperiphery length of a dot differs on different scanning lines, ascanning line which maximizes the periphery length of the dot is moreliable to cause unevenness on the recorded image due to a large imagevariation developed by the periphery length of the dot than otherscanning lines. Provided the periphery lengths of dots have the sameaverage value, such image unevenness is less visually perceptible if themaximum value of the periphery length of the dot is smaller. Statedotherwise, provided that dots are uniformly distributed, imageunevenness is less visually perceptible if the degree of a variation ofthe periphery length of the dot in the direction of the scanning linesis smaller.

A process of generating a dot where the degree of a variation of theperiphery length of the dot in the direction of the scanning lines issmaller will be described below.

For generating a dot where the degree of a variation of the dotperiphery length is smaller, i.e., a dot where the dot periphery lengthhas a smaller standard deviation σ, boundaries of dots (boundariesbetween blackened pixels and white pixels) should preferably be presentuniformly on each scanning line. Therefore, linearly aligned pixels arenot preferable.

For example, as shown in FIG. 11A, when a noticed pixel c3 is judgedaccording to a pattern matching process based on eight nearby pixelsincluding a dot 74 made up of five pixels arranged in an invertedL-shaped array, the noticed pixel c3 is determined as a candidateposition for a blackened pixel (a threshold to be determined this time).If the noticed pixel c3 is selected as a pixel candidate, then a dot 75made up of six pixels is produced as shown in FIG. 11B. The dots 74, 75have the same periphery length in the direction of scanning lines.However, the standard deviation σ of the periphery length in thedirection of scanning lines is smaller for the dot 75 than for the dot74.

Specifically, with respect to the dot 74, as shown in FIG. 11A, theperiphery lengths successively from the left along the respectivescanning lines are (1, 0, 2, 3), the average of the periphery lengths is1.5 (6/4), and the standard deviation σ of the periphery lengths isσ=[{(−0.5)²+(−0.5)²+(0.5)²+(1.5)²}/3]^(1/2)=1.29. With respect to thedot 75, as shown in FIG. 11B, the periphery lengths successively fromthe left along the respective scanning lines are (1, 1, 1, 3), theaverage of the periphery lengths is 1.5, and the standard deviation σ ofthe periphery lengths isσ=[{(−0.5)²+(−0.5)²+(−0.5)²+(1.5)²}/3]^(1/2)=1.0.

The process described above with reference to FIGS. 11A, 11B isillustrated as a relatively simply example. It is also possible toreduce the standard deviation σ according to a pattern matching processbased on eight nearby pixels or a pattern matching process based ontwenty-four nearby pixels.

FIG. 12A shows a characteristic curve 76 representing peripheral lengthratios in the direction of scanning lines (proportions by which scanningline lengths are held in contact with blackened pixels) in a dot patternhaving a dot percentage of 50% (a dot pattern 135 shown in FIG. 19E)according to the embodiment. FIG. 12B shows a characteristic curve 77representing peripheral length ratios in the direction of scanning linesin the dot pattern 2 (see FIG. 25) of the conventional 2×2-pixel FMscreen where the dot percentage is 50%.

It can be seen that the characteristic curve 76 representing peripherallength ratios in the dot pattern according to the present embodimentsuffers less variations than the characteristic curve 77 representingperipheral length ratios in the conventional dot pattern. The standarddeviation σ of the periphery length ratios is σ=0.00123 according to theprocess of the embodiment, and σ=0.0179 according to the conventionalprocess.

FIG. 13 shows a characteristic curve 78 of the standard deviation σ ofthe periphery length ratios of the dot pattern 2 of the conventional2×2-pixel FM screen and a characteristic curve 80 of the standarddeviation σ of the dot pattern 135 according to the present embodiment,the characteristic curves 78, 80 being plotted by determining aperiphery length ratio for each scanning line at each dot percentage andplotting the standard deviation σ of the periphery length ratio at eachdot percentage.

It can be seen from FIG. 13 that the standard deviation σ of theperiphery length ratios, which is indicative of the degree of variationsof the periphery length ratio for each scanning line, should preferablybe 0.019 or less, or more preferably be 0.018 or less, for all the dotpercentages.

By determining the standard deviation σ of the periphery length ratiosat the 19 dot percentages at intervals of 5%, it is possible to graspthe standard deviation σ of the periphery length ratios which representsvariations of the periphery length ratio in the direction of scanninglines at all the dot percentages.

With respect to a dot pattern 82 which is five pixels long in thedirection of scanning lines in FIG. 11A, for example, periphery lengthratios for the four central scanning lines are calculated as (0.2, 0,0.4, 0.6) successively from the left. Since the average of theseperiphery length ratios is 0.3, the standard deviation a of theperiphery length ratios in the direction of scanning lines is given asσ=[{(−0.1)²+(−0.3)²+(0.1)²+(0.3)²}/3]^(1/2)=0.258.

With respect to a dot pattern 84 which is five pixels long in thedirection of scanning lines in FIG. 11B, periphery length ratios for thefour central scanning lines are calculated as (0.2, 0.2, 0.2, 0.6)successively from the left. Since the average of these periphery lengthratios is 0.3, the standard deviation a of the periphery length ratiosin the direction of scanning lines is given asσ=[{(−0.1)²+(−0.1)²+(−0.1)²+(0.3)²}/3]^(1/2)=0.20.

If the relationship of the accumulated value Ds of the number Dn of dotswith respect to the dot percentage is established according to the curvenc shown in FIG. 4, then an increase in the dot gain in the intermediatetone areas is made smaller than with an FM screen where the accumulatedvalue Ds is established according to the curve na. In addition, asufficient resolution is provided in all the range of dot percentages aswith the conventional FM screens. If the number Dn of new dots in theintermediate tone areas is not increased according to the curve nc,i.e., is unduly reduced, then each dot becomes so large as to makegrainness visible, lowering the quality of images, with the result thatthe pattern frequency of the dot pattern becomes coarse.

Specifically, even if the number Dn of new dots is established at eachdot percentage according to the curve nc, when the dot percentageexceeds 25%, adjacent dots start contacting each other, and theaccumulated value Ds of the number Dn of dots according to the curve ncis not reached.

Actually, therefore, as indicated by the dot-and-dash-line curve nbwhich represents the accumulated value of the number of new dots in FIG.4, the number Dn of new dots of a minimum size is established such thatit increases again a substantially constant number after the dotpercentage exceeds 25% and until it reaches 50%. According to the curvenb, dots are prevented from contacting each other in the vicinity of thedot percentage of 50%, thus avoiding the occurrence of a tone jump.

In the dot percentage range from 50% to 100%, the accumulated value Dsof the number Dn of new dots may be established according to a curvewhich is in symmetric relation to the curves nc, nb with respect to thevertical line at the dot percentage of 50%. In the dot percentage rangefrom 50% to 100%, the curve is analyzed from 100% toward 50%, and thenumber of new-dots of white pixels (2×2 white pixels) is consideredrather than the number Dn of new dots of blackened pixels.

A process of determining thresholds th alternately successively inascending and descending orders in the highlight area HL and the shadowarea SD in step S4 will be described below with reference to a flowchartshown in FIG. 14. For the sake of brevity, the process of successivelydetermining thresholds th in the highlight area HL will mainly bedescribed below. In the shadow area SD, the same process of successivelydetermining thresholds th is carried out.

In step S11, the initial values of thresholds th_hl {0 through(thmax−1)/2} in the highlight area (0% through 50%) and thresholds th_sd{thmax through (thmax−1)/2} in the shadow area (100% through 50%) aredetermined to be th_hl=0, th_sd=thmax, respectively.

In the flowchart shown in FIG. 14, positions (array) for placing allthresholds th up to the dot percentage of 50% are determined in theorder of threshold 0→threshold thmax→threshold 1→threshold thmax−1→ . .. →threshold (thmax−1)/2.

For determining an array (placement positions) of thresholds th_hl inthe highlight area, dot center positions are established in step S12. Instep S12, dot center positions of the number Dn of new dots determinedin step S3 for the dot percentages, among the dot candidate positions inthe highlight area HL of the blackened portion (region) of the binarydata A2_bin (see FIG. 3D) determined in step S2, are established.

As described in Japanese Laid-Open Patent Publication No. 8-265566, thedot center positions are determined such that the dots established(assigned) by the thresholds th_hl whose placement positions are to bedetermined in the present threshold matrix TM are established inpositions most spaced from the presently existing dots determined by thethresholds th_hl−1 for the preceding gradation where the placementpositions of the thresholds th in the threshold matrix TM have alreadybeen determined.

For an easier understanding, the process will be described withreference to FIG. 15 which shows a super-threshold matrix STM made up ofnine threshold matrixes TM1 through TM9 each having 25 thresholds. Whenpositions for placing thresholds are determined in an ascending orderfrom the highlight areas HL of the threshold matrixes TM or in adescending order from the shadow areas SD thereof, central positions ofnewly placed thresholds th_hl are determined such that the alreadydetermined positions for placing thresholds th (“1” in FIG. 15) and thepositions for newly placing thresholds th_hl (“2” in FIG. 15) are mostspaced from each other in the threshold matrixes TM including a centralthreshold matrix (a 5×5 threshold matrix in FIG. 15) TM5 and otherthreshold matrixes TM1 through TM4, TM6 through TM9 of the samethreshold layout which are disposed around the central threshold matrixTM5 as nine nearby threshold matrixes in FIG. 15.

In the example shown in FIG. 15, the central threshold “2” in athick-line frame which is disposed within the threshold matrix TM5 isplaced in either a position which contains a point contacted by fourcircles around respective four thresholds “1” in thick-line framespositioned around the central threshold “2” or a position which iscloset to the above position and represents a blackened portion of thebinary data A2_bin (see FIG. 3D).

Specifically, as shown in FIG. 16A, positions 112 marked with Δ, forexample, in a dot pattern 110 which is made up of dots 108 based on thethresholds th determined up to present are determined as centralpositions for placing dots.

Then, in step S13, candidates (threshold candidates) th′_hl forpositions for placing thresholds are established. In this case, 2×2(n=4)-pixel dots of a minimum size determined in step Si around thecentral positions for placing dots which are determined in step S12 areestablished (placed), and used as candidates for placing new thresholds,i.e., threshold candidates th′_hl.

Specifically, as shown in FIG. 16B, the threshold candidates th′_hl forthe 2×2 (n=4)-pixel dots are set in the dot placing positions 112 markedwith Δ in FIG. 16, thus producing a dot pattern 114.

Then, in steps S14 through S16, it is determined whether the totalnumber of pixels of a dot pattern generated by the threshold matrixes TMwhere the layout of the thresholds th is determined up to present,corresponds to the present dot percentage or not, thereby correcting thetotal number of pixels. The dot pattern is generated as follows: Theimage data generator 12 generates continuous-tone image data (image dataI for generating a screen tint) of a gray pattern (whose pixel valuesare the same) corresponding to the dot percentage. The comparator 16compares the generated continuous-tone image data with the thresholdmatrixes TM stored in the threshold matrix storage unit 14 and includingthresholds up to the threshold th−1 which have been determined up topresent. Binary data H produced from the comparator 16 are supplied tothe dot pattern generator 18, which produces dot pattern data Ha. A dotpattern based on the dot pattern data Ha is displayed on the displayunit 20 b or the like.

In step S14, it is determined whether a present pixel count th_hl_totalwhich is the sum of the total number of pixels based on the thresholds 0through th−1 whose placement positions have already been determined andthe total number of pixels based on newly established thresholdcandidates th′_hl, is smaller than a required pixel countth_hl_num=N×N×th/thmax required at the present dot percentage or not(th_hl_total<th_hl_num).

If the present pixel count th_hl_total is smaller than the requiredpixel count th_hl_num, then since it is necessary to add as many pixelsas the difference (th_hl_num−th_hl_total) which is produced bysubtracting the present pixel count th_hl_total from the required pixelcount th_hl_num, new threshold candidates th′ are established as dotsfor adding those pixels from the dots that are not based on the existingthresholds 0 through th−1 or the dots that are not based on the newlyestablished threshold candidates th′_hl whose placement positions havenot yet been determined in step S15.

If the present pixel count th_hl_total is greater than the requiredpixel count th_hl_num, then since it is necessary to delete as manypixels as the difference (the present pixel count th_hl_total−therequired pixel count th_hl_num), dots for deleting those pixels areselected and deleted from the dots based on the newly establishedthreshold candidates th′_hl in step S16.

In step S16, of the dots making up the dot pattern, several dots maypossibly be smaller than dots of a minimum size. In the presentembodiment, because the dots of a minimum size are 2×2-pixel dots, thetotal number of pixels of the dot pattern which is made up of the dotsof a minimum size is a multiple of 4. If the total number of dots isadjusted in order to equalize dot percentages, 3-pixel dots, 2-pixeldots, or 1-pixel dots, which are produced by deleting one, two, or threepixels from each of 2×2-pixel dots, may be necessary.

In step S15, as disclosed in Japanese Laid-Open Patent Publication No.2001-292317, a dot pattern (binary image data) in the spatial domain,which is made up of the dots based on the thresholds 0 through th−1whose placement positions have already been determined and the dotsbased on the newly established threshold candidates th′_hl is FFTed intoa dot pattern in the frequency domain by the FFT unit 32, after whichhigh frequencies in the dot pattern are cut off by an LPF (Low-PassFilter) 40. Then, the dot pattern is IFFTed back into a dot pattern inthe spatial domain by the IFFT unit 36, after which low-frequencycomponents are extracted from the dot pattern. Positions where theextracted low-frequency components are weakest are set to thresholdcandidates th′ to be added. However, if a dot pattern having a dotpercentage of 50% is established in step S2, then positions where thelow-frequency components are weakest within blackened pixels of the dotpattern having the dot percentage of 50% may be set to thresholdcandidates th′ to be added.

A process of extracting positions where low-frequency components areweakest will be described below in greater detail. When a dot pattern isFFTed into a dot pattern in the frequency domain, since frequencycomponents present in the repetitive frequency of the threshold matrixTM are noise components (low-frequency components), the dot pattern isfiltered by the LPF 40 to extract the low-frequency components.

Since the noise components are perceived by the human being, thelow-frequency components are extracted by a human visual characteristicfilter 42, used as the LPF 40, which has a sensitivity level of 0 at aspatial frequency of 0 c/mm, a maximum sensitivity level of 1 in thevicinity of a spatial frequency of 0.8 c/mm, a sensitivity level ofabout 0.4 at a spatial frequency of 2 c/mm, and a sensitivity level ofabout 0 at a spatial frequency in the range from 6 to 8 c/mm. A model ofhuman visual frequency characteristics is described in detail in “Designof minimum visual modulation halftone patterns” written by J. Sullivan,L. Ray, and R. Miller, IEEE Trans. Syst. Man Cybern., vol. 21, No. 1,33-38 (1991).

Then, the low-frequency components extracted by the LPF 40 are IFFTedinto low-frequency components in the spatial domain by the IFFT unit 36.Because the produced low-frequency components have intensity variations,an image made up of these low-frequency components and the positions ofthe threshold candidates th′ in the threshold matrix TM are comparedwith each other, and positions where the low-frequency components areweakest (the values are smallest) are set to threshold candidatesth′_hl.

In the shadow area SD, positions where the low-frequency components arestrongest (the values are greatest) may be set to threshold candidatesth′_sd.

In step S16, low-frequency components may similarly be extracted, andpixels may be deleted from dots in positions where the low-frequencycomponents are strongest (the values are greatest), of the new thresholdcandidates th′_hl. In the shadow area SD, pixels may be deleted fromdots based on the new thresholds th′_sd in positions where thelow-frequency components are weakest (the values are smallest).

FIG. 17A shows a dot pattern 120 having a dot percentage of 30% wherethe dots of a minimum size are 2×2-pixel dots, according to the presentembodiment, the dot pattern 120 being generated by the above process.FIG. 17C shows a dot pattern 122 of the conventional 2×2-pixel dot FMscreen.

FIGS. 17B and 17D show dot patterns 124, 126, respectively, with darkand light areas which are produced by processing the dot patterns 120,122 with the visual characteristic filter 42 used as the LPF 40. FIGS.18A and 18B show in perspective respective patterns 128, 130 whichrepresent the dot patterns 124, 126 with dark and light areas. In FIGS.18A and 18B, the vertical axis represents dot percentages with white at0, black at 1.0, and the dot percentage of 30% at 0.30, and thehorizontal axes represent pixels. It can be seen that the dot pattern120 shown in FIG. 17A according to the present embodiment has smallerintensity variations in the dark and light areas and hence smalleramplitudes than the conventional dot pattern 122 shown in FIG. 17C.

In step S15 or S16, as disclosed in Japanese Laid-Open PatentPublication No. 2002-368995, when the dot pattern is IFFTed by the IFFTunit 36 to produce the low-frequency components in the spatial domain,the low-frequency components may further be FFTed by the FFT unit 32,and particular frequency components may be extracted in a descendingintensity order by a particular frequency component extractor 44. Theextracted particular frequency components may be IFFTed in a descendingintensity order to produce images in the spatial domain, and positionswhere intensity components are weakest, of the positions which do notintensify these images, may be set to threshold candidates th′ orthreshold candidates th′_hl.

According to the above processing in steps S12 through S16, apredetermined number of thresholds th may be established on thethreshold matrix TM corresponding to positions where dots are newlyassigned on the dot pattern.

In step S17, the dot pattern generated by the determined thresholds this optimized. This process of optimizing the dot pattern is not requiredif a high-quality dot pattern has been generated by the processing up tostep S16.

The process of optimizing the dot pattern may be either one or both ofthe method disclosed in Japanese Patent No. 3400316 and the processdisclosed in Japanese Laid-Open Patent Publication No. 2002-369005.

Specifically, according to the method disclosed in Japanese Patent No.3400316, low-frequency components are extracted from the dot patterngenerated by the thresholds th_hl. Of the extracted low-frequencycomponents, pixels that are placed in positions where the intensity isstrongest and pixels that are placed in positions where the intensity isweakest are switched around such that the former pixels will be whitepixels and the latter pixels will be blackened pixels, thereby reducingthe intensities of the low-frequency components. The blackened pixelshave to be pixels attached to the periphery of dots, i.e., pixels heldin contact with the periphery of dots, and the threshold th of theblackened pixels is of value equal to the threshold th of the dots.

According to the process disclosed in Japanese Laid-Open PatentPublication No. 2002-369005, as with the process disclosed in JapaneseLaid-Open Patent Publication No. 2002-368995, the dot pattern generatedby the thresholds th is FFTed, thereafter filtered by the visualcharacteristic filter 42 and the LPF 40, and then IFFTed intolow-frequency components in the spatial domain. The low-frequencycomponents are FFTed to extract frequency components in a descendingintensity order. The extracted particular frequency components areIFFTed in a descending intensity order to produce images in the spatialdomain, and pixels in positions where intensity components are weakest,of the positions which do not intensify these images and pixels that areplaced in positions where the intensity is weakest are extracted andswitched around, thereby reducing the intensities of the low-frequencycomponents. The extracted pixels have to be pixels attached to theperiphery of dots, and the threshold th of the blackened pixels is ofvalue equal to the threshold th of the dots.

In the process of extracting low-frequency components in steps S14through S17, as disclosed in Japanese Laid-Open Patent Publication No.2002-369005, a density image corresponding to a dot pattern output froman image output apparatus may be simulated, i.e., predicted, by adensity image simulator (predictor) 46, and low-frequency components maybe extracted from the density image. In this case, a test pattern isactually output from the output system 22, and the density imagesimulator 46 measures how one dot of the original dot pattern is outputon the test pattern with dark and light areas, thereby calculating thedot percentage of a density image close to an actual density image fromthe dot pattern.

An amount of exposure from the shape of the laser beam used in theoutput system 22 is integrally calculated, and a density image ispredicted from the gamma characteristics of the photosensitive materialon the printing plate materials EM.

The prediction of a density image based on calculations will bedescribed in detail below. A simulation shape for computer calculationsof a laser beam for forming 1×1-pixel dots, 2×2-pixel dots, . . . on arecording medium such as a film F or the like is determined. The laserbeam has a shape close to the Gaussian distribution which cansubstantially be expressed using a beam diameter that is determined bythe maximum value 1/e² of the amplitude. The amount of exposure for eachdot is calculated from the laser beam and the dot pattern.

Then, the amounts of exposure for the respective dots, i.e., 1×1-pixeldots, 2×2-pixel dots, . . . are converted into densities of the dotsusing the exposure characteristics, i.e., the gamma characteristics, ofthe photosensitive material such as a film or the like. A density image(density-simulated image) is obtained from the densities of the dotsthus determined. Low-frequency components can be extracted from thedensity image according to the above process using FFT. Actually,low-frequency components that are extracted from a density image canoften be more effective to remove noise components, rather thanlow-frequency components extracted from a dot pattern.

In this manner, the positions of thresholds th_hl in the thresholdmatrix are determined.

Then, in step S18, the newly established thresholds th_hl are set tothresholds th_hl+1 for the next gradation (th_hl=th_hl+1).

Similarly, thresholds th_sd for the shadow area SD are determined insteps S22 through S28.

In step S29, the thresholds th_hl determined from the highlight area HLand the thresholds th_sd determined from the shadow area SD are comparedwith each other for magnitude, and thresholds th_hl and thresholds th_sdare determined until they are of the same value, i.e., until the dotpercentage of 50% is achieved. When thresholds th_hl and thresholdsth_sd are of the same value, the generation of the threshold matrix isfinished.

FIGS. 19A through 19F show dot patterns 131 through 135, 137,respectively, which are part of dot patterns having dot percentages of10%, 20%, 30%, 40%, 50%, and 70% that are finally generated by dotpattern generator 18 by comparing the thus generated threshold matrix TMwith continuous-tone image data of gray patterns having correspondingdot percentages with the comparator 16.

The dot pattern 137 having the dot percentage of 70% may be a patternthat is generated by reversing the white and black areas of the dotpattern 133 having the dot percentage of 30%, or an independentlygenerated pattern.

The dot patterns 131 through 135, 137 shown in FIGS. 19A through 19F aregenerated by selecting the dot-and-dash-line curve nb which representsthe accumulated value of the number of new dots in FIG. 4. The dotpattern 131 having the dot percentage of 10% is made up of only2×2-pixel dots of a minimum size. The dot pattern 132 having the dotpercentage of 20% include a reduced proportion of 2×2-pixel dots of aminimum size and pixels (including 4- through 12-pixel dots)corresponding to the dot percentage that are attached to the peripheryof the existing dots (2×2-pixel dots). In dot percentages from 25% to30%, dots of a minimum size are not newly assigned, but pixels areattached to the existing dots, thereby increasing the blackened ratio.In dot percentages of 35% and higher, more dots of a minimum size arenewly assigned. Since the newly assigned dots serve to forcibly joinadjacent dots, the junctions between the dots can be distributed. Withthe above settings, it is possible to generate a threshold matrix TMcapable of generating a dot pattern for smoothly reproducing gradations.

According to the above embodiment, as described above, dots of a minimumsize, each made up of a certain number of pixels (one or more pixels),are determined for a highlight area, and a pattern frequency r in theintermediate tone of a dot pattern is determined (step S1). Based on thepattern frequency r, candidate positions for the dots are determined(step S2). Then, the number Dc of new dots of a minimum size isestablished at each dot percentage (step S3). Under the limitations ofthe number Dc of new dots of a minimum size and the pattern frequency rin the intermediate tone, thresholds th for generating optimum dotpatterns at respective dot percentages are successively generated (stepS4). In this manner, a threshold matrix TM optimum for the output system22 can be generated. The threshold matrix TM optimum for the outputsystem 22 means a threshold matrix TM which is capable of generating animage where dots are reliably and solidly assigned to a highlight area,and grainness is reduced and a dot gain is small in an intermediate tonearea, for example.

In the above embodiment, when the output system 22 has an outputresolution R pixels/mm and when dot pattern data Ha generated fromcontinuous-tone image data I whose pixel values correspond to a dotpercentage P of 50% as binary data H have a pattern frequency r c/mm, athreshold matrix TM having a matrix size of N×M pixels (including thecase where N=M) for converting a continuous-tone image into a dotpattern representing a binary image has a certain threshold array. Thethreshold array makes it possible to generate dot pattern data Ha wheredots of a minimum size which are made up of n pixels (n is at least 1)are provided out of contact with each other when the dot percentage Pincreases from 0% to a value where the number of dots corresponding tothe pattern frequency r becomes nearly N×M/(R/r)². Also, the thresholdarray makes it possible to generate dot pattern data Ha where pixels areattached to the periphery of the existing dots of a minimum size and thenumber of dots is not increased for dot percentages P after the numberof dots corresponding to the pattern frequency r becomes nearlyN×M/(R/r)².

In the dot percentages P after the number of dots corresponding to thepattern frequency r becomes nearly N×M/(R/r)², the threshold matrix TMhas the dot areas adjusted by attaching pixels to the periphery of theexisting dots of a minimum size.

The optimum size N×M, which will hereinafter be assumed to be N×N for abetter understanding, of the threshold matrix TM will be describedbelow.

The size N×N of the threshold matrix TM can be normalized, or statedotherwise can be evaluated, based on how many times the patternfrequency r c/mm is repeated in a dot pattern where the threshold matrixTM corresponds to the dot percentage of 50%. If the pattern frequency rc/mm is smaller, i.e., if the threshold matrix TM has less dots in thedot pattern, then the threshold matrix TM is considered to require alarger size.

A normalization threshold side size Nr is defined as an index forindicating how many times the pattern frequency r (also referred to as abasic frequency component r) is repeated in a single threshold matrixTM.

The normalization threshold side size Nr is expressed by the followingequation (4):Nr=N/(R/r)=N×r/R  (4)where N represents the size of one side of the threshold matrix TM (thenumber of pixels), R the resolution of the output system (pixels/mm),and r the pattern frequency (c/mm).

The normalization threshold side size Nr is thus defined as a valueproduced by dividing the size N (pixels) of one side of the thresholdmatrix TM by a pattern frequency repetition value (R/r) per outputresolution which virtually indicates how many times the basic pattern ofthe pattern frequency r is repeated at the output resolution R.

For example, if the pattern frequency r=20 c/mm, the output resolutionR=100 pixels/mm, and the size N of one side of the threshold matrix TMis N=200, then the normalization threshold side size Nr is calculated asNr=200×20/100=40.

Since the size of the threshold matrix TM is represented by N×N, as manybasic patterns of the pattern frequency r as the square of thenormalization threshold side size, i.e., Nr×Nr=40×40=1600, are containedin the size N×N of the threshold matrix TM. The square of thenormalization threshold side size Nr is referred to as normalizationthreshold size Nr×Nr.

A process of evaluating unevenness visibility of the generated thresholdmatrix TM will be described below.

As described above, a threshold matrix TM where dots of a minimum sizeare 2×2-pixel dots is generated at a pattern frequency r=20 c/mm and anoutput resolution R=100 pixels/mm, and the size N of one side of thegenerated threshold matrix TM is changed. Then, a dot pattern having adot percentage of 50% is generated from the threshold matrixes TM ofdifferent sizes N×N, and then FFTed by the FFT unit 32. This dot patternis represented by dot pattern data Ha generated as follows: The imagedata generator 12 outputs continuous-tone image data I of a size N×Nhaving pixel values corresponding to the dot percentage of 50%, and thecomparator 16 compares the pixel values of the continuous-tone imagedata I with the thresholds th of the threshold matrixes TM of the sizesN×N output from the threshold matrix storage unit 14 to produce binaryimage data H. The dot pattern generator 18 generates dot pattern data Hacorresponding to the binary image data H.

When the dot pattern, i.e., the dot pattern data Ha, is FFTed, theabsolute values of coordinates (μ, ν)=(r/N, 0): 90° component (afrequency component at 90° in the spatial domain, simply referred to as90° component), coordinates (0, r/N): 0° component, coordinates (r/N,r/N): 135° component, and coordinates (r/N, −r/N): 45° component of thebasic frequency in the frequency domain, which are indicated by soliddots (or black dots) on the grid pattern in FIG. 20, are obtained.

FIG. 21 shows a curve 140 representing the sum of the absolute values ofthe above four components plotted as indicating unevenness visibility(unevenness evaluation value).

In FIG. 21, the horizontal axis represents the normalization thresholdside size Nr. In this embodiment, the normalization threshold side sizeNr multiplied by the pattern frequency repetition value (R/r=5)represents the size N of one side of the threshold matrix TM.

It can be seen from the curve 140 that as the normalization thresholdside size Nr increases, unevenness visibility decreases, and changes inunevenness visibility are small when the normalization threshold sidesize Nr has a value near 70.

It can also be seen from the curve 140 that when a threshold matrix TMwhere the output resolution R is close (±10%) to 100 pixels/mm and thepattern frequency fr has a peak value close to about 20 c/mm isgenerated so as to have 2×2-pixel dots as dots of a minimum size, thenthe unevenness due to the repetition of the threshold matrix TM cannotbe essentially visually perceived if the normalization threshold sidesize Nr is greater than 65.

Since the unevenness reducing ability is not available when thenormalization threshold side size Nr increases to a certain level, it ispreferable that the normalization threshold side size Nr do not exceed75 also in view of the burden posed by the binary data processing.Actually, if color images are to be reproduced by printing, then thenumerical value of the normalization threshold side size Nr should notexceed 75 for the sizes of threshold matrixes TM for the respective C,M, Y, K printing plates. When there are some limitations such ashardware limitations, it is effective to use a threshold matrix sizewhich meets the requirement that the normalization threshold side sizeNr be in the range from 65 to 75, for only a color plate which makesunevenness easily noticeable, e.g., the K plate.

The above limitation on the threshold matrix size is effective for a dotpattern where the pattern frequency r is established for theintermediate tone area and there are dots having a relatively small dotperiphery length. This is because such a dot pattern has less freedomfor dot pattern optimization than the conventional threshold matrixwhere the dot periphery length is reduced in the intermediate tonelength and the pattern frequency r is not limited.

To summarize, according to an example, with respect to a thresholdmatrix TM for converting a continuous-tone image into a dot patternrepresenting a binary image, when the minimum number of pixels making updots of the dot pattern is 2×2, the reference periphery lengthproportion Ref_sur per unit area of the dot pattern is defined asRef_sur=(4×r×Q^(1/2))/R where r represents the pattern frequency, Q theblackened ratio, and R the output resolution, and the periphery lengthproportion determined per unit area for the dot pattern generated by thethreshold matrix TM corresponding to the blackened ratio Q isrepresented by Mes_sur, the threshold array of the threshold matrix TMshould preferably be determined such that a dot pattern is generatedwhere the dot pattern periphery length evaluation index Mes_sur/Ref_surdoes not exceed 1.14 for all blackened ratios Q ranging from 0 to 1.Also, the threshold array of the threshold matrix TM should preferablybe determined such that the normalization threshold side size Nr whichserves as an index indicative of how many times the pattern frequency ris repeated in one threshold matrix TM is defined as Nr=N×r/R where Nrepresents the size (the number of pixels) of one side of the thresholdmatrix TM, and the normalization threshold side size Nr is of an valuegreater than 65.

According to another example, with respect to a threshold matrix TM forconverting a continuous-tone image into a dot pattern representing abinary image, when the reference periphery length proportion Ref_sur perunit area of the dot pattern is defined as Ref_sur=(4×r×Q^(1/2))/R wherer represents the pattern frequency, Q the blackened ratio, and R theoutput resolution, and the periphery length proportion determined perunit area for the dot pattern generated by the threshold matrix TMcorresponding to the blackened ratio Q is represented by Mes_sur, thethreshold array of the threshold matrix TM should preferably bedetermined such that a dot pattern is generated where the dot patternperiphery length evaluation index Mes_sur/Ref_sur does not exceed 1.14for all blackened ratios Q ranging from 0 to 1. Also, the thresholdarray of the threshold matrix TM should preferably be determined suchthat the normalization threshold side size Nr which serves as an indexindicative of how many times the pattern frequency r is repeated in onethreshold matrix TM is defined as Nr=N×r/R where N represents the size(the number of pixels) of one side of the threshold matrix TM, and thenormalization threshold side size Nr is of an value greater than 65, butnot in excess of 75.

The threshold matrix TM has been illustrated as being square orrectangular in shape. Generally, an image is drawn when the recordinghead including the exposure unit 26, etc. is moved to scan the recordingmedium. If the repeated units of the threshold matrix TM, i.e.,superthreshold matrixes 142, and the scanning directions of therecording head are aligned with each other, e.g., if the superthresholdmatrixes 142 are arranged in a block pattern (square block pattern)where they are laid out in alignment with a main scanning direction MSand an auxiliary scanning direction AS (see FIG. 22), then unevenness isconsidered to be intensified in the units of the superthreshold matrixes142 which represent the repeated units of the threshold matrix TM. It iseffective to use the repeated units of the threshold matrix TM, i.e.,the superthreshold matrixes 142, as square or elongate rectangularshapes inclined through a rational angle.

A square superthreshold matrix 144 which is made up of nine thresholdmatrixes TM and inclined through tan θ=⅓ as shown in FIG. 23A isdeformed into a superthreshold matrix 146 shaped as a so-called Utahblock which comprises two large and small squares (the area of a hatchedportion and the area of a cross-hatched portion are equal to eachother), as shown in FIG. 23B.

The superthreshold matrix 146 shaped as a Utah block may be used toproduce a repetitive pattern 148 of stacked Utah blocks, as shown inFIG. 23C. The repetitive pattern 148 comprises a staggered stack ofsuperthreshold matrixes 146 each comprising two large and small squares.According to the repetitive pattern 148, the direction in which thesuperthreshold matrixes 146 representing the repeated units of thethreshold matrix TM are arranged and the scanning directions (the mainscanning direction MS and the auxiliary scanning direction AS) aredifferent from each other, unevenness caused by the superthresholdmatrixes 146 representing the repeated units of the threshold matrix TMis reduced.

FIG. 23D shows a repetitive pattern 150 of elongate rectangularsuperthreshold matrixes. As can be seen from FIG. 23D, the repetitivepattern 148 of Utah blocks shown in FIG. 23C may be composed of therepetitive pattern 150, which is a so-called brick block pattern, wherethe superthreshold matrixes 146 are arranged successively from No. 1.

The repetitive pattern 148 of Utah blocks shown in FIG. 23C may beconsidered to include basic repeated units of the repetitive pattern 150of elongate rectangular superthreshold matrixes, the basic repeatedunits being shifted each time they are scanned in the auxiliary scanningdirection AS.

If a simultaneous multi-channel image recording system based on themulti-beam exposure principle shown in FIG. 1 is employed for increasingthe rate at which an image is recorded on the printing plate materialsEM, then the size of the threshold matrix TM or the size of thesuperthreshold matrix STM should preferably be of a value different fromthe number of channels of the simultaneous multi-channel image recordingsystem. This is because if the size of the threshold matrix TM or thesize of the superthreshold matrix STM is the same as the number ofchannels of the simultaneous multi-channel image recording system, thenunevenness caused by the multiple beams and unevenness caused by thethreshold matrix TM or the superthreshold matrix STM are added to eachother, making pitch unevenness easily visually recognizable as noisecomponents.

In the above description, only one printing plate has been described.For reproducing color images, however, it is customary to employ a7-color printing process including separated C, M, Y, K colors and R, G,B colors, or a 6-color printing process including C, M, Y, K colors, Gcolor, and orange color. Though different threshold matrixes having mthreshold matrix sizes may be generated with respect to m (m>4) colors,any interference between complementary colors is small as dotpercentages for complementary colors are hardly increased. Therefore, athreshold matrix for a color may also be used for its complementarycolor. For example, when inks of C, M, Y, K colors and R, G, B colorsare used, one threshold matrix may be used for M and G printing plates,one threshold matrix for C and R printing plates, and one thresholdmatrix for Y and B printing plates. Similarly, when inks of C, M, Y, Kcolors, G color, and orange color are used, one threshold matrix may beused for M and G printing plates, and one threshold matrix for C andorange printing plates.

Threshold matrixes thus generated may be used as follows:

FIG. 24 shows a printing/platemaking system 200 incorporating thresholdmatrixes-generated by the threshold matrix generating apparatus 20 ofthe threshold matrix generating system 10 shown in FIG. 1.

In the printing/platemaking system 200, RGB image data captured by adigital camera 202 as an image capturing unit or RGB image data (or CMYKimage data) read by a plate input machine 204 as a scanner (imagereader) are supplied to an RIP (Raster Image Processor) 206, whichconverts the RGB image data into CMYK image data.

The RIP 206 stores in its storage unit such as a hard disk or the likedata of threshold matrixes TM (threshold matrix data) generated by thethreshold matrix generating apparatus 20 and supplied through an opticaldisk 208 serving as a storage unit such as a CD-R or the like or througha communication link.

The RIP 206 compares the CMYK image data and the corresponding CMYKthreshold matrix data with each other, and converts the CMYK image datainto CMYK dot pattern data (CMYK image data).

The CMYK dot pattern data are then sent to a DDCP (Direct Digital ColorProofer) 210, which produces a print proof PRa on a sheet of paper. TheDDCP 210 allows the operator to confirm noise components and printingquality on the print proof PRa before the image data are processed by aprinting press 220. The sheet of paper used by the DDCP 210 may be asheet of printing paper used by the printing press 220.

The RIP 206 delivers the CMYK dot pattern data to a color ink jetprinter 20c1 which produces a printing proof PRb on a sheet of paper ora color electrophotographic printer 20c2 which produces a printing proofPRc on a sheet of paper.

The CMYK dot pattern data are also sent to the exposure unit 26 whichserves as a filmsetter or a platesetter in the output system 22 such asa CTC apparatus or the like. If the exposure unit 26 is a filmsetter,the automatic developing machine 28 generates a film F. The film F issuperposed on a printing plate material, and exposed to light by aplanar exposure unit (not shown), producing a printing plate PP. If theexposure unit 26 is a platesetter as shown in FIG. 1, then the automaticdeveloping machine 28 directly outputs a printing plate PP. The exposureunit 26 is supplied with printing plate materials EM from a magazine 212of photosensitive materials (including plate materials).

CMYK printing plates PP are mounted on plate cylinders (not shown) in aK-plate printer 214K, a C-plate printer 214C, an M-plate printer 214M,and a Y-plate printer 214Y of the printing press 220. In the K-plateprinter 214K, the C-plate printer 214C, the M-plate printer 214M, andthe Y-plate printer 214Y, the CMYK printing plates PP are pressedagainst a sheet of printing paper supplied from a printing paper supplyunit 216 to transfer the inks to the sheet of printing paper, therebyproducing a printed material PM on which a color image is reproduced. Ifthe printing press 220 is configured as a CTC apparatus, then the RIP206 supplies the CMYK dot pattern data directly through a communicationlink, and the printing plates mounted on the plate cylinders are exposedto record image data and then developed directly into printing platesPP.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. A computer readable medium having stored therein a threshold matrix,said computer readable medium containing computer executableinstructions for: causing the threshold matrix to convert acontinuous-tone image into a dot pattern representing a binary image,wherein a minimum number of pixels making up dots of the dot pattern is2×2, and when a reference periphery length proportion per unit area ofthe dot pattern is defined as Ref_sur =(4×r×Q^(1/2))/R where rrepresents a pattern frequency, Q represents a blackened ratio, and Rrepresents an output resolution, and a periphery length proportiondetermined per unit area for the dot pattern generated by the thresholdmatrix corresponding to the blackened ratio Q is represented by Mes_sur,a threshold array of the threshold matrix is determined such that a dotpattern is generated where a dot pattern periphery length evaluationindex defined as Mes_sur/Ref_sur does not exceed 1.14 for all blackenedratios Q ranging from 0 to 1, and that a normalization threshold sidesize Nr which serves as an index indicative of how many times thepattern frequency r is repeated in one threshold matrix is defined asNr=N×r/R where N represents a size of one side of the threshold matrixas a number of pixels, and the normalization threshold side size Nr isof an value greater than
 65. 2. The program of claim 1 wherein thecomputer readable medium is a storage unit in a raster image processor.3. A computer readable medium having stored therein a threshold matrix,said computer readable medium containing computer executableinstructions for: causing the threshold matrix to convert acontinuous-tone image into a dot pattern representing a binary image,wherein when a reference periphery length proportion per unit area ofthe dot pattern is defined as Ref_sur=(4×r×Q^(1/2))/R where r representsa pattern frequency, Q represents a blackened ratio, and R represents anoutput resolution, and a periphery length proportion determined per unitarea for the dot pattern generated by the threshold matrix correspondingto the blackened ratio Q is represented by Mes_sur, a threshold array ofthe threshold matrix is determined such that a dot pattern is generatedwhere a dot pattern periphery length evaluation index defined asMes_sur/Ref_sur does not exceed 1.14 for all blackened ratios Q rangingfrom 0 to 1 is generated, and that a normalization threshold side sizeNr which serves as an index indicative of how many times the patternfrequency r is repeated in one threshold matrix is defined as Nr=N r/Rwhere N represents a size of one side of the threshold matrix as anumber of pixels, and the normalization threshold side size Nr is of anvalue greater than 65, but not in excess of
 75. 4. The program of claim3 wherein the computer readable medium is a storage unit in a rasterimage processor.
 5. A threshold matrix generating system comprising: athreshold matrix generator; and a threshold matrix storage unit forstoring a threshold matrix which converts a continuous-tone image into adot pattern representing a binary image, wherein when a referenceperiphery length proportion per unit area of the dot pattern is definedas Ref_sur=(4×r×Q^(1/2))/R, where r represents a pattern frequency, Qrepresents a blackened ratio, and R represents an output resolution, anda periphery length proportion determined per unit area for the dotpattern generated by the threshold matrix corresponding to the blackenedratio Q is represented by Mes_sur, a threshold array of the thresholdmatrix is determined such that a dot pattern is generated where a dotpattern periphery length evaluation index defined as Mes_sur/Ref_surdoes not exceed 1.14 for all blackened ratios Q ranging from 0 to 1 isgenerated, and that a normalization threshold side size Nr which servesas an index indicative of a number of times the pattern frequency r isrepeated in one threshold matrix is defined as Nr=N r/R, where Nrepresents a size of one side of the threshold matrix as a number ofpixels, and the normalization threshold side size Nr is of an valuegreater than 65, but not in excess of
 75. 6. The threshold matrixgenerating system according to claim 5, further comprising an outputsystem which forms the dot pattern generated by the threshold matrix onprintable media.